The present invention relates to a fuel-burner method and apparatus in which a stream of fuel is burned in two stages to inhibit NO.sub.x formation. More particularly, the present invention relates to such a a fuel-burning method and apparatus in which combustion of the fuel in a first of the two stages is supported by a first oxygen containing gas and combustion of the fuel is supported in a second of the two stages by a second oxygen-containing gas having a greater oxygen concentration than the first oxygen-containing gas.
Fuel burners are used in furnaces for producing thermal melts for a wide variety of industrial applications. Thermal melts can comprise ferrous and non-ferrous metals, glass, and etc. In order to maximize the power output of a burner, while at the same minimizing fuel consumption, the prior art has provided burners that are designed to oxidize the fuel in the presence of oxygen or oxygen-enriched air. The problem with such furnaces is that atmospheric nitrogen can react with oxygen to produce a noxious pollutant known in the art as thermal NO.sub.x. In addition, fuel radicals such as CH can react with atmospheric nitrogen to form prompt NO.sub.x. Moreover, in case of liquid fuels, fuel-bound nitrogen may form HCN which can oxidize to form fuel-bound NO.sub.x. This problem, which can arise even in those prior art furnaces wherein the oxygen necessary to support combustion is supplied from air, is the result of high combustion temperatures, large availability of fuel radicals and fuel-bound nitrogen in the flame.
In order to alleviate thermal NO.sub.x formation, prior art burners are designed to burn fuel in two stages (staged combustion). In a first stage of combustion, known in the art as the fuel-rich stage, combustion occurs in the presence of substoichiometric amounts of oxygen to lower combustion temperatures and thereby to inhibit thermal NO.sub.x formation. Downstream of the first stage, unburned fuel and combustible hydrocarbons are present. In a second stage of the combustion a combustible mixture of the hydrocarbons and unburned fuel burn in oxygen that is supplied from the same source that is used to support combustion in the first stage. However, in the second stage of combustion, the oxygen is introduced in superstoichiometric amounts to produce what is known in the art as a fuel-lean stage of combustion. The superstoichiometric amounts of oxygen are required to fully oxidize the combustible mixture produced in the first stage of combustion. It is to be noted that the fuel fragments have a lower heat of formation, and as such, thermal NO.sub.x is not a major source of NO.sub.x formation in the second stage of combustion. However, incomplete as well as slow combustion of the combustible mixture in the second stage of combustion can result in high concentrations of hydrocarbon radicals which will react with nitrogen to eventually produce prompt NO.sub.x.
Since such prior-art burners utilize the same source of oxygen in both stages, that is air or oxygen or oxygen-enriched air enriched to the same extent in both stages, the difference in the spread between the stoichiometry in the first and second stages of combustion is limited. In this regard, a dimensionless ratio known in the art as equivalence ratio can be obtained by dividing a total amount of fuel by a total amount of oxygen present in any stage of combustion and dividing the result by a quotient of the theoretical amounts of fuel and oxygen that would be necessary to stoichiometrically support combustion. In a fuel-rich stage, the equivalence ratio is greater than 1.0 to indicate the excess of fuel. In the fuel-lean stage, the equivalence ratio is less than 1.0 to indicate the surplus of oxygen.
In the prior art, the maximum equivalence ratio that can be obtained in the fuel-rich stage is limited because a point is reached in which combustion will not be supported given the amount of oxidant being added. In other words, a flame in the fuel-rich stage will eventually not be able to be stabilized and will blow off. In addition, as the fuel-rich stage becomes richer, the fuel-lean stage needs more oxidant to complete combustion. In order to fully oxidize the combustible mixture and prevent prompt NO.sub.x while preventing a blow-off of the second stage flame due to large amounts of oxidant in the second stage, the equivalence ratio of the combustion in the second stage of combustion has to be preferably limited to near stoichiometric proportions. This is difficult to achieve in the case of air or oxygen-enriched air having the same oxygen concentration as in the first stage of combustion because the amount of oxygen-containing gas that is added to the second stage of combustion can act to cool the second stage of combustion and/or blow-off the first stage, thereby extinguishing the flame.
As will be discussed, the present invention provides a two-stage fuel-burning method and apparatus that inherently allows a greater equivalence ratio to be obtained in the first stage of combustion than in the prior art, and also, an equivalence ratio in the second stage of combustion that approaches unity. As a result, NO.sub.x suppression is enhanced over prior art combustion methods and apparatus.